By John Evans Special to Yahoo Sports

Every running back relies on the ability of his offensive line to open lanes for him. With horrendous run-blocking, even the most gifted of runners will be buried in the backfield more often than not. And any ordinary back can make hay when routinely sprung into the open field.

Whether or not the three running backs below exceed fantasy gamers’ expectations or disappoint may be decided by the performance of their lines. The fascinating but disconcerting part is that it’s hard to say what will happen. Injuries are always the most unpredictable variable, and each line has players whose health is critical.

Ezekiel Elliott, Dallas Cowboys

Entering the preseason the Cowboys’ o-line was the kind of unit that could guarantee Ezekiel Elliott success vs. stacked boxes – they were that overpowering. But now Dallas is dealing with the inherent fragility of offensive line play, as in the span of a few days they’ve gone from the gold standard to a group with significant concerns.

After Travis Frederick was manhandled in practice, something that simply doesn’t happen to the NFL’s best center, further testing was done and doctors found that he’s suffering from GBS (Guillain-Barré syndrome), a rare auto-immune disease that had a similar impact on Washington lineman Mark Schlereth in the early ‘90s. Medical science has advanced over the last two decades and they caught it early with Frederick, so it’s likely he’ll make a full recovery.

Optimistically, he could be back to normal in weeks, not months or years. However, this tenacious disease could sideline the center all season or render his play inconsistent if he rushes back. With a condition this serious, getting back on the football field is not the top priority for Frederick.

The Cowboys may have another problem area at left guard. Rookie Connor Williams has the potential to become an outstanding NFL player, if he can increase his functional strength. Unfortunately, the likely Week 1 starter has struggled in practice and preseason action. Another variable is Zack Martin’s knee injury. He returned to practice on Tuesday, but if the injury slows his performance or he suffers a setback, that’s a second former All-Pro with a question.

Fantasy GMs are counting on two things when selecting Elliott with a top five draft pick: 1) a massive workload and 2) a dominant offensive line that can compensate for a predictable offense with few credible threats at other positions. If the line is compromised, that eliminates the second part of the equation. He also faces one of the league’s least favorable schedules for running backs.

Elliott was a good, not great receiver at Ohio State who won’t suddenly become David Johnson out of the backfield, but an inability to run the ball at will and a paucity of pass-catching weapons could actually enhance his value in half or full-PPR formats if Zeke becomes the recipient of endless dump-off passes. It’s a possibility. In any case, a dysfunctional offensive line won’t produce the touchdown-scoring opportunities a top-five pick usually requires to deliver on draft cost.

We’re still a long way from this being a unit that would potentially torpedo Elliott’s prospects. The Cowboys’ OL corps is by no means bereft of talent. After six fairly undistinguished seasons Frederick’s understudy, Joe Looney, is admittedly a journeyman, but he should be adequate at least. Last year Tyron Smith’s injury fill-ins failed, undermining the basic functionality of the offense and contributing to the Cowboys’ fatal losing streak, but the center position isn’t as critical as left tackle and Frederick’s backup is not as significant of a step down.

We can’t be certain Smith, Martin and La’el Collins will be healthy entering Week 1. If the outlook continues to trend toward the optimistic for Smith and Martin when you draft, I would still take Elliott fifth overall, behind some combination of Todd Gurley, Le’Veon Bell, David Johnson and Antonio Brown. Were you to take Alvin Kamara there in PPR, I wouldn’t disagree at this point. If the news takes a turn for the worse, then Elliott should slide past Saquon Barkley and DeAndre Hopkins too, especially in PPR.

As a Charger, Gordon has never had good blocking. His critics point to his pedestrian yards-per-carry average and say he’s a J.A.G. whose fantasy value is derived from volume, as the only game in town on early downs. But Gordon was a touted prospect for a reason. According to PlayerProfiler.com, his college yards per carry (7.5) put him in the 95th percentile among RBs. Yes, Wisconsin’s line is generally excellent, but Gordon’s ranks in speed (72nd), burst (72nd) and agility (80th) mean that less than 30 percent of his peers could match or exceed his athleticism.

Now the Chargers’ line is poised to take a crucial step forward. They opened gaping holes against New Orleans this preseason and, for the first time in seemingly forever, should enter the regular season healthy. The addition of Miami’s Mike Pouncey via free agency has not only upgraded the center position, it’s an injection of hard-nosed attitude into the unit. The center is traditionally the pace-setter of a line and while Pouncey wasn’t the most durable Dolphin, his leadership was unquestioned.

The Chargers actually have depth and upside, as second-year guard Forrest Lamp is just now returning to game action after a torn ACL last preseason. Lamp was one of 2017’s top prospects but will have to displace a solid starter from Los Angeles’ interior. The current guards, Dan Feeney and Michael Schofield, are playing well.

While the Chargers’ 2017 grades aren’t sterling, the chemistry they’re developing and the edge Pouncey brings to the unit bode well for Gordon. He may not be the kind of back who can transcend the play of the line in front of him, but at the pro level we haven’t seen what he can do with a cohesive front five, either. Let’s hope a perennially snake-bitten team has better injury luck this season. If so, Gordon will be much more efficient with his carries and could challenge for the rushing title.

Derrick Henry, Tennessee Titans

The “cheapest” option of this trio, Henry doesn’t need to be a true bell cow to justify his draft position. But he is the very definition of an RB whose success hinges on that of his line. A big, lumbering back, Henry lacks the agility and stop-start acceleration to create his own yards. The line must give him the clear runway he needs to get up to speed, at which point he’s very hard to bring down. Running downhill on a regular basis, the former Alabama back could have a big year. Even splitting the workload evenly with Dion Lewis, Henry’s big-play ability and goal-line dominance might make him a weekly RB2.

The question is, will Henry get the blocking he needs? On paper Tennessee has an elite line. But the same group was nothing special last season and they’ve looked far from dominant in the preseason. Through three exhibitions, the Titans are averaging 3.1 yards per carry. It’s perplexing, because the team has made big investments in its line, both in terms of cash and draft capital. Guards Josh Kline and Quinton Spain have been no great shakes in the ground game, and right tackle Jack Conklin remains a candidate to open the season on the PUP list – that would cost him six games. On the other hand, if Conklin returns soon and picks up where he left off, it could lift this line back to top 10 status.

For all his upside, Henry is an iffy pick in fantasy drafts. There’s too much uncertainty surrounding his role and his offensive line.